Perhaps the keyboard's onboard chorus was used on the very lowest setting. It will have been fed through an analogue mixing desk if their studio setup environment was fixed in place (synths patched into the desk)... so then maybe some sends were used for getting the signal into an effects rack... Phaser and aural exciter, perhaps.

Then maybe they EQ's the channel quite heavily and also EQ'd the return to emphasize harmonics.

Perhaps they then multi-sampled the piano notes, sequenced it, applying further processing on the Akai, EMU, Roland or Yamaha sampler. In doing that, and saving the multi samples to disk, they would have had a "hard copy" of the song's instrument for remixes or whatever. So this "creating core sounds", getting them onto the sampler, and then using them in a sequence/performance is a totally different way of working than VST plugins etc. Remember that most of these 90s producers started in the late 80s with only a sampling keyboard, a drum machine and a Roland synth so sampling was part of their natural work-flow.

Also; the piano notes might well have been a "performance" (multiple triggered patches) and not just a single instrument... at any stage of the production (i.e pre-sampled or post-sequenced)

Oh , I'll add this: Depending on if/how they multi-sampled the piano, when they spanned the samples across the keys, it would have formed certain harmonic character due to how multi-sampling intrinsically works.

So, for example, Let's say you sampled C four times on four octaves, then spanned it, (also keep in mind that the Roland source synth contains multi samples in ROM), when you then play notes, off the sampler, within just one octave then you'll get a certain harmonic character yet when the riff ascends into the next octave (different sample) the resulting harmonic character changes because the polyphony is now being filled by two simultaneous samples.

This way of working produces different harmonics than all the sounds coming off a single CPU!

Sound on Sound have some "lost art of sampling" articles that probably explain it way better than my ramblings do !

One of the best ways to humanize it is to use Swing (in FL - not sure about other DAW's).
Drop me a message on Facebook, I can send you my FL preset for m1 piano and hopefully it's what you are looking for - fb.com/keshofficial

I'm using the m1 from the Korg Legacy collection in a few WIPs. I find I don't EQ it much, just a hipass a little higher than normal to cut back the fundamental frequency a bit. That helps bring out those odd highs, which is what I want it for. Other than that, I find I sequence it a bit differently to get it to do what I want. I have to add more notes in the sub octave to create more of those higher harmonics for me to interact with (I basically copy/paste my chord down an octave in addition to the normal notes). As far as humanizing it goes, that's all in the sequencing IMO. Swing baby swing!